Heretofore, the Fe—Ni alloy materials have been used as various functional materials including a magnetic material, a lead frame and a shadow mask. These materials are worked to a product thickness of about 0.1–1 mm in accordance with applications. Particularly, Fe-36 wt % Ni alloy is low in the thermal expansion coefficient and is useful as a shadow mask material. This shadow mask material is usually produced subjecting a Fe—Ni alloy sheet to an etching treatment using an aqueous solution of ferric chloride.
As to the etching workability of the shadow mask material, there are many inventions in view of surface properties (JP-A-4-99152 and so on), plane orientation (JP-A-1-247558 and so on) and the like. Also, study examples focusing attention to non-metallic inclusions contained in the alloy are disclosed in JP-A-61-84356 and JP-A-7-268558, but all of them aim only at the reduction of the non-metallic inclusion amount. However, even if the non-metallic inclusion amount is reduced, there may be a case that the deterioration of hole shape accompanied with the poor etching work is caused depending upon the kind and composition of the non-metallic inclusions.
That is, when holes are formed by the etching treatment using the aqueous solution of ferric chloride at the production step of the shadow mask, if non-metallic inclusions are existent in positions to be pierced and the etching is carried out, the shape of the holes in the shadow mask material is poor. Especially, if the non-metallic inclusions are soluble in the etching solution, the hole shape is further poor. Particularly, if the non-metallic inclusion is mainly composed of MgO or CaO, as shown in FIG. 1, the non-metallic inclusions existing on the surface of the thin sheet are dissolved in the etching solution and corrode Fe—Ni alloy therearound to cause a problem that the shape of the etched hole is deteriorated.
It is, therefore, an object of the invention to develop a technique capable of solving the above problems of the conventional technique and to provide a Fe—Ni alloy material for shadow mask having an excellent etching workability.